The present invention relates to a scanning optical system employed, for example, in a tandem type color laser beam printer.
Conventionally, a scanning optical system, which is employed, for example, in a tandem type color laser beam printer is known. Such a color laser beam printer includes a plurality of scanning optical systems and photoconductive drums corresponding to a plurality of color components of a color image formed by the color laser beam printer. In each scanning optical system, a laser diode, a polygonal mirror and an fθ lens are provided. The laser beam emitted by the laser diode is deflected by the polygonal mirror. The deflected laser beam is converged by the fθ lens and forms a beam spot on a surface to be scanned. Since the polygonal mirror is rotated, the deflected beam scans within a predetermined angular range. Thus, the beam spot formed on the surface to be scanned moves along a predetermined scanning line, extending direction of which will be referred to as a main scanning direction. The surface to be scanned is a circumferential surface of a photoconductive drum. By using the plurality of scanning optical systems and the corresponding photoconductive drums for a plurality of color components, respectively, a plurality of color image components are printed, in an overlaid fashion, on the same sheet so that a color image is formed.
In such a tandem type color printer, i.e., a printer employing a plurality of scanning optical systems and photoconductive drums, in order to avoid color drift of an image, writing start position and writing end position of each scanning line of each color component should be adjusted accurately.
However, if the fθ lens of each scanning optical system has lateral chromatic aberration, and wavelength of a laser beam emitted by each laser diode has individual errors, then the writing start and/or end positions of the scan line may be different among the color components. In such a case, the color drift appear on a printed image and the quality of the formed image is deteriorated.